The prior art has provided freeze dryers incorporating a pressure vessel having a freeze drying chamber for receiving sterile material to be freeze dried. The sterile material is held within a plurality of containers which are sealed by loosely fitted stoppers adapted to be mechanically driven into the containers after the material is freeze dried. The containers are stacked upon a collapsible stack of trays located within the freeze drying chamber and a pressure plate is provided to push the shelves and containers together in order to drive the stoppers into the containers. The pressure plate is in turn driven by an actuating arm of a double-acting hydraulic ram connected to the pressure vessel. Actuation of the hydraulic ram in one direction produces an extension of the actuating arm to press the pressure plate against the containers and shelves and actuation of the hydraulic ram in an opposite direction produces a retraction of the actuating arm in order to remove the containers from the shelves and the pressure vessel.
The sterility of the freeze drying chamber is maintained by a variety of well known methods. However, there is no assurance that the sterility of the hydraulic ram is maintained since extension and retraction of the actuating arm is effectuated by a hydraulic cylinder located outside of the freeze drying chamber. The hydraulic cylinder can coat the outer surface of the actuating arm with oil or other contaminants. As a result, the non-sterile surface of the actuating arm can introduce contaminants into the freeze drying chamber and can thereby contaminate the sterile material to be freeze dried.